Fuel assemblies for nuclear reactors and particularly for pressurized water nuclear reactors consist of a bundle of very long fuel rods which are held in a framework, in such a way that the rods are parallel to one another and are arranged in regular network in the transverse planes of the assembly perpendicular to its longitudinal direction.
The fuel rods are held by spacer grids uniformly spaced over the length of the assembly and forming elements of the framework.
The fuel assemblies of pressurized water nuclear reactors in operation at the present time have a length for over four meters and a square cross-section, the side of which has a length of approximately twenty centimeters on each side. The fuel assemblies intended for nuclear reactors of a power of 900 MW comprise eight spacer grids uniformly spaced over the length of the assembly, and the fuel assemblies intended for reactors of a power of 1300 MW have ten spacer grids. The outer lateral faces of the spacer grids project slightly in relation to the bundle of fuel rods of the assembly.
In order to transport the fuel assemblies, for example new fuel assemblies intended for refuelling the core of a nuclear reactor, containers are employed in which two fuel assemblies are arranged and locked during transportation.
The transport containers comprise lower and upper half-shells which can be assembled together by screwing, of which the lower half-shell of the container receives an underframe or cradle for supporting the fuel assemblies and of which the upper half-shell forms the cover of the container.
The supporting cradle of the fuel assemblies consists of an elongate structure, the T-shaped cross-section of which provides, for each of the fuel assemblies, two supporting surfaces at right angles which are arranged on either side of the middle part of the cradle forming a partition wall for the two assemblies.
The cradle rests within the lower half-shell of the container by means of shock-absorbing studs.
Each of the assemblies arranged in a container comes to bear against the cradle with two successive lateral faces at 90.degree. of its spacer grids. The clamping of the assemblies against the bearing surfaces of the cradle is ensured by flanges arranged level with each of the spacer grids of the assemblies.
In the prior art, level with each of the spacer grids of each of the fuel assemblies in the transport position on the cradle, two half-flanges are mounted in an articulated manner on the cradle so as to be capable of being turned down in the direction of the fuel assembly and of the supporting surfaces when the assembly has been fastened. The half-flanges in the turned-down position are connected to one another by means of a screw connection. Each of the half-flanges carries a device for clamping the assembly, consisting of a threaded rod engaged in the corresponding half-flange and carrying a bearing shoe at its end. The clamping of the assembly is ensured by bringing the shoes to bear, with some pressure, on the outer lateral surfaces of the spacer grids opposite the faces of these spacer grids bearing on the cradle, by means of a nut and a lock nut which are engaged on the threaded rod.
To ensure the clamping and flanging of the assembly on the cradle, it is therefore necessary to turn down each of the two half-flanges, to ensure the assembling together of the two half-flanges by clamping the screw connection and to bring the shoes to bear by torque-clamping of each of the nuts.
These operations, must be conducted for each of the spacer grids of the fuel assemblies, are relatively time-consuming and require the attendance of well-trained personnel, inasmuch as defective fastening of the assemblies occurring as a result of insufficient clamping can result in damage to the assemblies during their transportation. Likewise, excessive clamping of the shoes can cause damage to the spacer grids of the assemblies.
Moreover, the use of wrenches for clamping the screw connections and the nuts of the shoes entails some risk that these wrenches, which are of considerable mass, will fall onto the fuel assemblies and that the rods of the assemblies will therefore be damaged. Falling tools can also cause physical injury to the operators responsible for putting the fuel assemblies into the container.
It is clear that the clamping devices of the prior art also have similar disadvantages when the fuel assemblies are unloaded after a container has been opened. In fact, it is necessary to unclamp the shoes and the flanges before the cradle and the assemblies have been raised vertically, for example by use of a mechanical device for raising the underframes.
In more general terms, in many sectors of industry, it is necessary to transport heavy components inside containers or on transport underframes after effective clamping or flanging of the components.
This clamping of the components on their transport underframe is generally executed by using screw connections fastened to a flanging part of the structure opposite the bearing faces of the object to be transported.
The clamping torque of these screw connections has to be carefully controlled.
The corresponding operations are therefore relatively time-consuming and complex and usually require the attendance of highly competent personnel.